Test LWB buffer overflow for block cloning

PR#15634 removes 128K into 2x68K LWB split optimization, since it
was found to cause LWB buffer overflow while trying to write 128KB
TX_CLONE_RANGE record with 1022 block pointers into 68KB buffer,
with multiple VDEVs ZIL.

This commit adds a test for this particular scenario by writing
maximum sizes TX_CLONE_RANE record with 1022 block pointers into
68KB buffer, with two SLOG devices.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by: Umer Saleem <usaleem@ixsystems.com>
Closes #15672